1. Field of the Invention
This invention relates to a method for manufacturing intricate parts possessing good soft magnetic properties, using iron-nickel-phosphorous ternary alloy powders prepared by simultaneous electroless-plating of nickel and phosphorous onto the surface of iron powder or iron and nickel powder mixtures. The concentration of phosphorous is close to or higher than its maximum solubility in the solid solution of iron and nickel. When mixed with an appropriate amount of organic binder, these powders can be formed into intricate parts by standard metal injection molding procedures, which comprise of blending, granulation, injection molding, debinding, and sintering processes. Low sintering temperatures afford parts with high sintered density, isotropic sintered shrinkage, and a composite structure characterized by large spheroidal grains of solid solution of iron, nickel, and phosphorous imbedded in insulating iron-nickel phosphide. The magnetic properties thus achieved are substantially improved with respect to the prior practices.
2. Description of the Prior Art
Powder metallurgy process is very suitable for economical mass production of intricate soft magnetic components. Different materials, such as Fe--P, Fe--Ni, and Fe--Si alloys, are being produced via this route. The drawbacks of this process are, however, relatively poor and erratic magnetic characteristics of the final product, due to the existence of a large amount of residual porosity. For example, sintering of the mixture of Fe and Ni powders is difficult since sintering temperatures higher than 1300.degree. C., followed by post-sintering secondary repressing and thermal annealing, are required to enhance their magnetic performance.
An alternative approach to enhance material transfer is incorporating a metalloid element, such as phosphorous, to form a liquid phase during sintering. The Fe--P system has been promoted commercially in recent years as a new soft magnetic material, which is being claimed to be capable of replacing virtually all other high performance soft magnetic powder metallurgy materials. Nevertheless, phosphorous is generally added in the form of Fe.sub.3 P, causing microscopically non-uniform distribution of phosphorous element in the powder mixture. This results in existence of large pores and segregation of phosphorous subsequent to sintering. The optimum fraction of phosphorous is thus limited to 0.4 and 0.8 wt %, and, consequently, the effect of phosphorous on enhancement of sintered density is restricted. Increase in the phosphorous concentration can improve the uniformity of distribution of phosphorous in the powder mixture, but the major drawback of large sintered shrinkage rate, associated with high temperature sintering of Fe--P alloy system by press-and-sinter process, precludes this possibility. For example, Fe--0.8% P alloy is often used as an improved material beyond the popular Fe--0.45% P alloy, but it exhibits a greater sintered shrinkage and distortion in press-and-sinter practice. In such a case, a coining operation is often needed to control the dimensions of the part, and a subsequent low temperature thermal anneal is required to remove the surface deformation without altering the dimensional control, as the surface region is the only area that is magnetized and demagnetized in alternating magnetic fields.
As per a U.S. Pat. No. 5,505,760 issued in U.S. Patent Gazette dated Apr. 9, 1996, soft magnetic properties of powder processed iron based alloys are improved by using powders prepared by mixing SnP powder, or Sn and Fe.sub.3 P powders, with iron powder. However, again due to the microscopically non-uniform distribution of the alloying elements, the concentration of tin should be maintained higher than 4.5 wt % to be effective in improving the sintered density and, consequently, soft magnetic properties.
Eddy current losses play a major role in core loss in applications involving alternating magnetic fields. The above mentioned material systems possess low magnitudes of electrical resistivity and are used primarily in applications involving very low frequencies, as eddy current losses account for a large energy loss. In practical terms, only the electrical resistivity and sample thickness can be modified to minimize the eddy current losses. Hence, the strategies that have been devised to overcome these energy losses include constructing laminated composites composed of magnetic foils separated by insulating polymer films, and forming magnetic powder--polymer composites. However, these approaches suffer from degradation of magnetic inductance arising from the polymeric materials. Additionally, for powder processed materials, the thickness is not a parameter that can be used as a variable to control the eddy current losses. Electrical resistivity is therefore the only control variable that can be used in powder metallurgy to control the eddy current losses.
In a recent work (U.S. patent pending) by Materials Innovation Inc. (West Lebanon, N.H.), an electro-deposition coating technology has been developed, wherein the coating layer on the iron powder surface imparts lubrication during pressing and forms a highly electrically resistive compound encompassing each iron grains, subsequent to a low temperature sinter. The eddy current loss in an alternating magnetic field with frequencies ranging from 20 to 500 Hz is thus dramatically reduced. However, an electro-deposition coating process possibly suffers non-uniformity of thickness of the coated materials among the particles, arising from the inhomogeneous density of electric current, and thus requires sophisticated equipment to assure consistent quality of products.
Ternary Iron-nickel-phosphorous alloy powders have also been prepared by electroless plating. However, the intention of such approach is to enhance the sintered strength by incorporation of phosphorous, while simultaneously avoiding intergranular precipitation of brittle phosphides. In addition, coated composite powder with high concentration of phosphorous is very difficult to press due to its inherent high hardness. Thus, the phosphorous concentration in these ternary alloy powders is usually conservatively low (&lt;0.5 wt %).
Based on the above discussion, it is the objective of the present invention to provide a process wherein mass production of Fe--Ni--P ternary alloy powders can be tailored without the use of costly equipment, and intricate soft magnetic components with excellent magnetic properties, especially high saturation magnetization as well as high electrical resistivity, in an alternating magnetic field can be economically mass-produced.